题目网址: http://www.puzzleup.com/2014/ http://www.puzzleup.com/2014/puzzle/?245 答题时限: ８月21日７ＰＭ－比赛结束（约12月17日） 加分时限: ８月21日７ＰＭ－８月27日６：５９ＰＭ 答对可得基本分１００分。答案可上传５次，每改１次答案从基本分扣２０分。　 比赛期间内可随时上传答案，加分时限内答对第ｎ天加（６－ｎ）分　 　　 　　 另依题目的难易有额外加分（如有８０％的人这题答错，答对者加８０分）　　 ◆Four Letters You are asked to create a text using only the letters A, B, C, and D. If all possible four-letter strings (from AAAA to DDDD) are required to be included within the text, what should be the minimum length of this text? If the problem was asked for the letters A, and B, then the answer would be 5. Sample text:AABBA 你被要求只以 A, B, C, D 四个字母构成一串文字。 若所有的四字母组合都要出现 (从 AAAA 到 DDDD)，试问最小字串长度是多少？ 若题目改问两个字母 A, B, 则答案为 5。可能答案：AABBA。 [译注: 於是看起来是指所有 AAAA AAAB AAAC ... 到 DDDD 一共 256 种组合都要有] -- 'You've sort of made up for it tonight,' said Harry. 'Getting the sword. Finishing the Horcrux. Saving my life.' 'That makes me sound a lot cooler then I was,' Ron mumbled. 'Stuff like that always sounds cooler then it really was,' said Harry. 'I've been trying to tell you that for years.' -- Harry Potter and the Deathly Hollows, P.308 --

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